Rapamycin, a bacterial macrolide, is emerging as a promising agent for the treatment of cancer. The mammalian Target of Rapamycin (mTOR) controls translation of proteins for growth, proliferation, and progression of the cell cycle. We hypothesize that mTOR may have rapamycin-independent functions as well. Using a model of mTOR knockdown by RNA interference (RNAi) in breast cancer cells, our specific aims are as follows: 1. To determine functional effects of mTOR knockdown and 2. To obtain a comprehensive molecular profile of mTOR knockdown. To address the first aim, we will measure proliferation using cell-viability assays, assess cell size using particle counters, and check apoptosis using caspase and TUNEL assays. To address the second aim, we will use Western analysis to examine known downstream targets of mTOR, and gene-expression arrays to obtain global expression profiles. We anticipate finding significant differences between mTOR knockdown by RNAi and mTOR inhibition by rapamycin, which would lead to further investigation of rapamycin-independent mTOR functions. Such functions may prove to be novel targets in cancer chemotherapy, particularly in the setting of rapamycin-resistance.